1. Technical Field
This invention relates to a shoe for swash plate type compressors used for automobile air conditioners and a shoe assembly.
2. Prior Art
A swash plate type compressor, as schematically shown in FIG. 7, includes a rotary shaft 3 rotatably supported in a pair of cylinder blocks 5a, 5b, said rotary shaft 3 having a swash plate 2 fixed thereon. The cylinder blocks 5a, 5b are formed with a plurality of circumferentially equispaced cylinder bores 5, each bore 5 having a double ended piston 4 slidably received therein. Each piston 4 is centrally formed with a recess 4a in such a manner as to straddle the outer periphery of the swash plate 2, said recess 4a having spherical seats 4b formed at the axially opposite surfaces thereof. A shoe 1 is received in each spherical seat 4b and interposed between the swash plate 2 and the piston 4, serving to smoothly transform the rotary motion of the swash plate 2 into the reciprocating motion of the piston 4. While the double acting type having double ended pistons has been taken up by way of example, the single acting type has basically the same construction.
The shoe 1 is generally hemispherical and engages the spherical seat 4b of the piston 4 through its spherical outer peripheral surface and slidably contacts the swash plate 2 through its flat bottom surface.
There has heretofore been known an arrangement wherein, as shown in FIGS. 8 and 9, of the spherical outer peripheral surface of the shoe 1, the contact portion which contacts the spherical seat 4b and the skirt portion are constructed with mutually different radii of curvature (see Japanese Patent Publication Hei 3-51912). FIGS. 8 and 9 show the spherical seat 4b of the piston 4 and the shoe 1 received therein, and the spherical seat 4b is defined by a single radius of curvature, while the spherical outer peripheral surface of the shoe 1 comprises a top reference surface 1a having substantially the same radius of curvature as the spherical seat 4b and a retracted spherical surface 1b formed by retracting the spherical band portion of the skirt from the reference spherical surface 1a toward the center of the shoe 1, said spherical band portion of the skirt repeating engagement with and disengagement from the spherical seat 4b. In other words, the curvature of the retracted spherical surface 1b differs from that of the reference spherical surface 1a and furthermore the respective curvatures of the various portions of the retracted spherical surface 1b gradually change, which means that the outer peripheral surface of the shoe 1 has a plurality of curvatures. As a result, a suitable clearance which gradually increases from the boundary between the retracted spherical surface 1b and the reference spherical surface 1a variously changes its size according to the swing movement of the shoe 1 and assists in the wedge action, enabling effective feeding of lubricating oil to the contact region of the sliding reference spherical surface 1a.
In swash plate type compressors, the shoe is forced to perform the so-called precession according to the rotary swing movement of the swash plate, a fact which is liable to cause local contact, which, in turn, causes other drawbacks, such as local wear. Therefore, in order to ensure correct contact of the shoe, it is necessary to control the shoe during the manufacture thereof so that the position of contact of the shoe is within a predetermined range.
In the case where the outer peripheral surface of the shoe which contacts the spherical seat of the piston has substantially the same radius of curvature as that of the spherical seat, the position of contact tends to vary rather than remain unchanged. Furthermore, the position of contact of the shoe with the spherical seat is influenced by the quality of finish of the spherical surface of the shoe, a fact which makes it difficult to control the height of the shoe. Further, in the case where the outer peripheral surface of the shoe is defined by aplurality of curvatures, it is difficult to process the shoe and the position of contact of the shoe with the spherical seat of the piston is more liable to vary.